1. Field of the Invention
The present invention relates to a controller for a machine tool including a feed shaft motor driving a feed shaft and main shaft motors driving main shafts and, in particular, to a controller that implements power failure protection for a machine tool including two main shafts moving in synchronization with each other through a workpiece held by the main shafts.
2. Description of the Related Art
In many machine tools including a feed shaft motor and a main shaft motor, the main shaft is used as a drive source for driving a main shaft to which a workpiece or a tool (any of various types of tools), for example, is attached and the feed shaft motor is used as a driving source for driving a feed shaft that moves the main shaft, for example. In a motor controller for such a machine tool, AC power input from an AC input side is first converted to DC power, then further converted to AC power, which is then used as drive power for driving the motors each of which is provided for each of the shaft. Such a machine tool is equipped with, as main circuits of a motor controller, a converter which converts (rectifies) AC power supplied from the AC power supply side at which a three-phase AC input power supply is located and outputs DC power and inverters which are connected to a DC link (direct-current link), which is the DC side of the converter, and interconverts DC power from the DC link and AC power which is drive power for driving the motors or regenerative power generated by the motors. The controller controls an AC output from each of the inverters to a desired voltage and a desired frequency to control the speed, torque or rotor position of each of the main shaft motor and the feed shaft motor connected to the AC side of each inverter.
The inverter is provided for each of the motors which drive multiple driving shafts (the feed shaft and the main shaft). Because of a demand for energy saving, regenerative inverters are often used to store regenerative power generated during motor deceleration in an electric storage device provided on the DC link and reuse the regenerative power as power for driving the motors or provide the power back to the AC power supply side.
Due to a demand for energy saving as with the inverters, a regenerative converter is sometimes used that is capable of providing regenerative energy generated during motor deceleration back to the AC power supply side.
When a power failure occurs on the AC power supply side of the converter of the motor controller described above, the feed shaft motor and the main shaft motor in the motor controller may not continue proper operation. In that case, a collision of the feed shaft can cause some problems such as damage or deformation of a motor, the motor controller that drives the motor, a tool connected to the motor driven by the motor controller, a workpiece to be machined with the tool, the manufacturing line that includes the motor controller, or other equipment.
To prevent a collision of the feed shaft due to a power failure on the AC power supply side, operation of the feed shaft motor that drives the feed shaft needs to be stopped as soon as possible. For that purpose, a power failure determination unit is provided on the AC power supply side of the converter to monitor a power failure on the AC power supply side and, when a power failure occurs, a deceleration command is provided to the feed shaft motor to stop the feed shaft motor, thereby avoiding or minimizing the kind of problem described above to protect the main shaft moved by the feed shaft motor and a tool connected to the main shaft or a workpiece to be machined by the tool. When the power supply for a computer unit of the controller is backed up by an uninterruptible power source device (UPS) or the like, even when a power failure occurs on the AC power supply side, the controller can provide a command to the inverter for the feed shaft motor to cause an action to be taken in a time of emergency, the inverter for the feed shaft motor can be kept operating for a while with charge stored in a capacitor provided in the converter and the feed shaft motor can be immediately stopped.
However, when a deceleration command is provided to the feed shaft motor in response to detection of a power failure on the AC power supply side to immediately stop the feed shaft motor, the regenerative power may not be provided back to the AC power supply side during the power failure and, as a result, DC voltage on the DC link between the converter and the inverters will increase. This is especially remarkable when regenerative power of the motor is large. To address the problem, when DC voltage on the DC link, which is the DC side of the inverter, excessively increases, the inverter typically issues an “overvoltage alarm” in order to protect the inverter itself and discontinues control.
Depending on characteristics of the feed shaft motor or the degree of friction on the feed shaft driven by the feed shaft motor, the feed-shaft-motor inverter needs to continue supplying drive power to the feed shaft motor even when the feed shaft motor is to be decelerated. In other words, in this case, since regenerative power is not generated in the feed shaft motor, the feed-shaft-motor inverter does not supply energy to the DC link but, on the contrary, the feed-shaft-motor inverter converts DC power from the DC link to AC power and supplies the AC power to the feed shaft motor even though the feed shaft motor is decelerated. In such a situation, when a power failure occurs on the AC power supply side and a deceleration command for emergency stop is provided as described above, the DC voltage on the DC link would rapidly decreases. When the DC voltage on the DC link, which is on the DC side of the inverter, excessively decreases, the inverter usually may not supply drive power and therefore issues a “low-voltage alarm” to discontinue control.
To avoid occurrence of such an “overvoltage alarm” and “low-voltage alarm” which would occur due to variations in DC voltage on the DC link caused by deceleration of the feed shaft motor, in some cases the DC voltage on the DC link is monitored and, when the DC voltage increases, the main shaft motor is accelerated to consume the increase in the DC power on the DC link which has caused the increase in the DC voltage on the DC link to hold down the increase in the DC voltage; on the other hand, when the DC voltage on the DC link drops, the main shaft motor is decelerated to produce regenerative power to compensate for the decrease in the DC power on the DC link which has caused the DC voltage drop on the DC link to suppress the decrease in the DC power on the DC link. Hereinafter, the operation of suppressing variations in the DC power on the DC link by accelerating or decelerating the main shaft motor depending on a DC voltage value on the DC link in the event of a power failure is referred to as the “power failure backup operation”. Since the power failure backup operation can suppress variations in the DC voltage values on the DC link even when the operation of the feed shaft motor which drives the feed shaft is quickly stopped in order to prevent a collision of the feed shaft in the event of a power failure on the AC power supply side, occurrence of an “overvoltage alarm” and a “low-voltage alarm” can be avoided.
In a machine tool that includes multiple main shafts and two of the main shafts hold one workpiece, the two main shafts are coupled together through the held workpiece so that they can move in concert with each other. In this case, when a power failure occurs on the AC power supply side of the converter of the motor controller, the synchronization between the two main shafts maintained by the held workpiece is lost and the main shafts independently decelerate to stop, which may twist the workpiece resulting breaking or deforming the workpiece.
To avoid such a twist of a workpiece, there is a technique that when a power failure occurs on the AC power supply side of a machine tool in which two main shafts are coupled by holding a workpiece, a chuck of one of the main shafts that have held the workpiece is loosen and the workpiece is held only by the other main shaft while the two main shafts are decelerated to stop, thereby preventing the workpiece from twisting, as described in Japanese Laid-open Patent Publication No. 2001-105209, for example.
When the power-failure backup operation described above is applied to two main shafts that are coupled through a held workpiece and move in concert with each other in a machine tool that includes multiple main shafts, two of which hold one workpiece, the two main shafts move in synchronization with each other through the held workpiece and the synchronization is maintained before occurrence of a power failure on the AC power supply side (i.e., during a normal operation) but once a power failure occurs, the two main shafts would independently perform power failure backup operations. The power failure backup operations performed independently by the two main shafts make a difference in the conditions of acceleration/deceleration operations of the main shaft motors, which causes the synchronization between the two main shafts maintained by the held workpiece to be lost to twist the workpiece, thereby causing a problem such as breakage or deformation.
To avoid a twist of a workpiece in a machine tool to which such a power failure backup operation is applied, the invention described in Japanese Laid-open Patent Publication No. 2001-105209 may be further applied. However, the operation of loosening the chuck of one of the main shafts in a power failure needs to be completed instantaneously and a mechanical arrangement for accomplishing such an operation would be complicated. Additionally, in a power failure, control would be needed to be performed for both of the power-failure backup operation and the operation of loosening the chuck of one of the main shafts, which inevitably adds to complexity of the control system.